In Situ Growth of CdZnS Nanoparticles@Ti3C2Tx MXene Nanosheet Heterojunctions for Boosted Visible-Light-Driven Photocatalytic Hydrogen Evolution

Using natural light energy to convert water into hydrogen is of great significance to solving energy shortages and environmental pollution. Due to the rapid recombination of photogenerated carriers after separation, the efficiency of photocatalytic hydrogen production using photocatalysts is usually very low. Here, efficient CdZnS nanoparticles@Ti3C2Tx MXene nanosheet heterojunction photocatalysts have been successfully prepared by a facile in situ growth strategy. Since the CdZnS nanoparticles uniformly covered the Ti3C2Tx Mxene nanosheets, the agglomeration phenomenon of CdZnS nanoparticles could be effectively inhibited, accompanied by increased Schottky barrier sites and an enhanced migration rate of photogenerated carriers. The utilization efficiency of light energy can be improved by inhibiting the recombination of photogenerated electron-hole pairs. As a result, under the visible-light-driven photocatalytic experiments, this composite achieved a high hydrogen evolution rate of 47.1 mmol h−1 g−1, which is much higher than pristine CdZnS and Mxene. The boosted photocatalytic performances can be attributed to the formed heterojunction of CdZnS nanoparticles and Ti3C2Tx MXene nanosheets, as well as the weakened agglomeration effects.


Introduction
Energy shortages and environmental pollution are the main problems facing the world today [1,2]. Hydrogen, a clean and carbon-free natural energy source with a high energy density, can aid in the better resolution of energy and environmental issues [3][4][5]. Currently, hydrogen is used in many fields, such as the oil industry, the new energy industry, and so on. As the demand for hydrogen continues to increase, the scientific community is also exploring ways to increase hydrogen production [6][7][8]. Since Fujishima and Honda discovered the phenomenon of photocatalytic water splitting in 1972 [9], scientists have been working on this technology. Photocatalytic hydrogen production, as a green and pollution-free technology that can convert solar energy into hydrogen energy, has now become one of the most promising research directions.
In the past decade, metal sulfides have been widely employed in various fields, such as optical imaging and energy storage applications [10][11][12], due to their excellent electronic, optical, and semiconductor properties. Transition metal sulfides, such as WS 2 , MoS 2 , ZnS, and CdS, are widely cited in the field of photocatalysis due to their suitable band gaps [13]. Ganapathy et al. [14] prepared amorphous SrTiO 3 -crystalline PbS heterojunctions that exhibited a hydrogen production performance of 5.9 mmol h −1 g −1 under ultraviolet light negative charge of Ti 3 C 2 T x causes electrostatic adsorption, so cadmium and zinc ions could be tightly attached to Ti 3 C 2 T x , which effectively inhibited the agglomeration of CZS nanoparticles. In this way, the efficient separation and utilization of electrons and holes can be realized. As a result, a kind of photocatalyst with more efficient hydrogen production performance can be obtained.

Synthesis of Photocatalysts
The CdZnS/Ti 3 C 2 T x photocatalyst was synthesized by the method shown in Figure 1. Solution A: 2 mmol of cadmium acetate hydrate and 2 mmol of zinc acetate hydrate were dissolved in 4 mL of deionized water, and then an appropriate amount of Ti 3 C 2 T x was added to the above solution. Finally, the mixture was stirred for 1 h. Solution B: 5 mmol of thioacetamide (TAA) was added to 4 mL of deionized water, then dissolved by stirring. Solution C: 8 mmol NaOH was dissolved in 2 mL of deionized water. In a typical synthesis, Solution B was added dropwise to solution A and stirred for about 1 h. Then, 2 mL of 4 mol L −1 NaOH aqueous solution was added, drop by drop, in the stirred state after stirring for another hour. The mixture was hydrothermally treated at 180 • C for 24 h. In this work, CdZnS samples with different ratios of Ti 3 C 2 T x were named CZS-1, CZS-2, CZS-2.5, CZS-3, and CZS-5, where the numbers represent the mass percentage of added Ti 3 C 2 T x (1%, 2%, 2.5%, 3%, and 5%). The preparation process of the original CZS is similar to that of the CZS@Ti 3 C 2 T x composite photocatalyst, with the difference being the absence of Ti 3 C 2 T x in the precursor solution of the original CZS.  [42][43][44]. Inspired by this, we successfully constructed the heterojunction composite CZS/Ti3C2Tx by a hydrothermal in-situ growth and assembly method in this work. The negative charge of Ti3C2Tx causes electrostatic adsorption, so cadmium and zinc ions could be tightly attached to Ti3C2Tx, which effectively inhibited the agglomeration of CZS nanoparticles. In this way, the efficient separation and utilization of electrons and holes can be realized. As a result, a kind of photocatalyst with more efficient hydrogen production performance can be obtained.

Synthesis of Photocatalysts
The CdZnS/Ti3C2Tx photocatalyst was synthesized by the method shown in Figure 1. Solution A: 2 mmol of cadmium acetate hydrate and 2 mmol of zinc acetate hydrate were dissolved in 4 mL of deionized water, and then an appropriate amount of Ti3C2Tx was added to the above solution. Finally, the mixture was stirred for 1 h. Solution B: 5 mmol of thioacetamide (TAA) was added to 4 mL of deionized water, then dissolved by stirring. Solution C: 8 mmol NaOH was dissolved in 2 mL of deionized water. In a typical synthesis, Solution B was added dropwise to solution A and stirred for about 1 h. Then, 2 mL of 4 mol L −1 NaOH aqueous solution was added, drop by drop, in the stirred state after stirring for another hour. The mixture was hydrothermally treated at 180 °C for 24 h. In this work, CdZnS samples with different ratios of Ti3C2Tx were named CZS-1, CZS-2, CZS-2.5, CZS-3, and CZS-5, where the numbers represent the mass percentage of added Ti3C2Tx (1%, 2%, 2.5%, 3%, and 5%). The preparation process of the original CZS is similar to that of the CZS@Ti3C2Tx composite photocatalyst, with the difference being the absence of Ti3C2Tx in the precursor solution of the original CZS.

Characterization
The structure of the prepared photocatalyst was characterized by X-ray diffraction (XRD, PW3040/60, PANalytical, Almelo, The Netherlands) with a scanning range of 10-80°. The micromorphology, lattice stripe, and bonding information of the fabricated photocatalyst heterojunction interface were analyzed using field emission scanning electron microscopy (SEM, JSM-7001F, JEOL, Akishima, Japan) and field emission transmission

Characterization
The structure of the prepared photocatalyst was characterized by X-ray diffraction (XRD, PW3040/60, PANalytical, Almelo, The Netherlands) with a scanning range of 10-80 • . The micromorphology, lattice stripe, and bonding information of the fabricated photocatalyst heterojunction interface were analyzed using field emission scanning electron microscopy (SEM, JSM-7001F, JEOL, Akishima, Japan) and field emission transmission electron microscopy (TEM, Talos-F200S, FEI, MA, USA). The valence bands and elemental compositions of the photocatalysts were investigated using X-ray photoelectron spectroscopy (XPS, K-Alpha Nexsa, Thermal Science, USA). The light absorption performance of the photocatalyst was analyzed with a UV-VIS spectrophotometer (UV-VIS, UV-3600Plus, Shimadzu, Japan) in the range of 200 to 800 nm. The photoluminescence intensity of the samples at an excitation wavelength of 375 nm was determined using a fluorescence spectrometer (PL, FS5, Edinburgh Instruments, Livingston, UK).

Photoelectrochemical Measurement
Electrochemical impedance spectroscopy (EIS) and photocurrent response tests were performed using a standard three-electrode system at room temperature. A homogeneous suspension was formed by adding 10 mg of the sample to a mixture of 200 µL ethanol and 20 µL Nafion solution, followed by sonication for 30 min. The suspension was lowered onto the surface of cleaned tin fluoride oxide (FTO)-doped glass. Subsequently, the substrate was dried at 60 • C for 12 h. For recording photoresponse signals, a 300 W Xe lamp (with a UV cutoff filter, λ > 420 nm) was used as the light source. The frequency range of the electrochemical impedance spectroscopy (EIS) measurement was 10 5 Hz to 0.1 Hz.

Photocatalytic H 2 Evolution Performance
The photocatalytic hydrogen evolution reaction was conducted on an all-glass automated on-line trace gas analysis system (Labsolar-6A, PerfectLight, Beijing, China). A 10 mg quantity of photocatalyst was evenly dispersed in 100 mL of 0.35 M Na 2 S/0.25 M sodium sulfite aqueous solution and stirred using sonication for 30 min. The 300 W Xe lamp is equipped with a 420 nm cutoff filter (λ > 420 nm). Before illumination, the system was evacuated to eliminate the influence of impurities, which were detected using a gas chromatograph (GC9790II, FuLi, Zhejiang, China. TCD detector, and Ar as the carrier gas). Hydrogen production per hour was recorded. The apparent quantum yield (AQY) was measured under similar photocatalytic conditions. The difference is that AQY was measured using a 420 nm bandpass filter and the optical power density was measured using an irradiometer. AQY was calculated according to Equation (1) [45]:

Results and Discussion
The XRD patterns of the as-prepared samples are shown in Figure 2. As can be seen, the CZS with an amphibolite structure had a distinct characteristic peak. According to the standard card, it can be indexed to the standard hexagonal phase cadmium sulfide (JCPDS 41-1049) and cubic phase zinc sulfide (JCPDS 05-0566). In contrast to them, half of the Cd atoms were replaced by Zn atoms, indicating the formation of a homogeneous solid-phase solution. It was not a mixture of cadmium sulfide and zinc sulfide on the physical layer. The peaks of the CZS/Ti 3 C 2 T x composite photocatalyst at 26.5 • , 44.8 • , and 52.3 • can be ascribed to the (111), (220), and (311) planes of CZS, respectively, which is consistent with the XRD peak positions of CZS in previous literature [28]. The diffraction peaks of Ti 3 C 2 T x were consistent with previous literature reports [46]. Compared to cubic zinc sulfide (JCPDS 05-0566), CZS exhibited the polyphase characteristics of the hexagonal phase, with a slight shift towards lower angles. This indicates that increasing the zinc content in the CZS solid solution transformed the crystal phase from hexagonal cadmium sulfide to cubic zinc sulfide, suggesting that the formed CZS was a solid solution rather than a physical mixture of cadmium sulfide and zinc sulfide. Compared with the original CZS, no significant shift was observed on the CZS/Ti 3 C 2 T x composite photocatalyst, which indicates that the recombination of CZS and Ti 3 C 2 T x did not change the crystal orientation of CZS. The absence of Ti 3 C 2 T x in the diffraction pattern was mainly due to its low quantity and CZS covering the surface of Ti 3 C 2 T x .
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to analyze the morphology and microstructure of the samples. The SEM and TEM images of CZS and CZS-2 are shown in Figure 3. The samples were initially synthesized as agglomerated nanoparticles (Figure 3a,d). Ti 3 C 2 T x has a unique layered structure ( Figure 3c). As shown in Figure 3b, it can be seen that the CZS-2 sample had a relatively Nanomaterials 2023, 13, 2261 5 of 13 obvious structure of particles attached to the flake. CZS nanoparticles were covered on the flake Ti 3 C 2 T x . Thus, the structure had a larger contact area and stronger photoadsorption capacity. Moreover, more light reflection and light scattering on the uneven surface of the hybrid layers also improved the utilization rate of light. In addition, the gap between the layers also facilitated the entry of the sacrifice agent. Figure 3e shows two different lattices, typically 0.24 nm corresponding to the (103) plane of Ti 3 C 2 T x , and 0.32 nm corresponding to the (111) plane of CZS. The coexistence of these two lattices indicates the successful synthesis of the CZS/Ti 3 C 2 T x heterojunction photocatalyst. The uniform distribution of C, S, Zn, Cd, and Ti elements on the energy dispersive X-ray spectroscopy (EDX) mappings indicates the uniform coverage of CZS nanoparticles on Ti 3 C 2 T x (shown in Figure 3f). The similarity of the molar ratios of each element (Figure 3g) to the experimental sample further verifies the successful preparation of the sample. absence of Ti3C2Tx in the diffraction pattern was mainly due to its low quantity and CZS covering the surface of Ti3C2Tx. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to analyze the morphology and microstructure of the samples. The SEM and TEM images of CZS and CZS-2 are shown in Figure 3. The samples were initially synthesized as agglomerated nanoparticles (Figure 3a,d). Ti3C2Tx has a unique layered structure ( Figure 3c). As shown in Figure 3b, it can be seen that the CZS-2 sample had a relatively obvious structure of particles attached to the flake. CZS nanoparticles were covered on the flake Ti3C2Tx. Thus, the structure had a larger contact area and stronger photoadsorption capacity. Moreover, more light reflection and light scattering on the uneven surface of the hybrid layers also improved the utilization rate of light. In addition, the gap between the layers also facilitated the entry of the sacrifice agent. Figure 3e shows two different lattices, typically 0.24 nm corresponding to the (103) plane of Ti3C2Tx, and 0.32 nm corresponding to the (111) plane of CZS. The coexistence of these two lattices indicates the successful synthesis of the CZS/Ti3C2Tx heterojunction photocatalyst. The uniform distribution of C, S, Zn, Cd, and Ti elements on the energy dispersive X-ray spectroscopy (EDX) mappings indicates the uniform coverage of CZS nanoparticles on Ti3C2Tx (shown in Figure 3f). The similarity of the molar ratios of each element (Figure 3g) to the experimental sample further verifies the successful preparation of the sample. The surface composition and chemical valence of CZS-2 were analyzed by X-ray photoelectron spectroscopy (XPS). As shown in Figure 4a, S, Zn, Cd, C, and other elements could be detected on CZS-2. In comparison with the original CZS, slight shifts in the peaks of CZS-2 could be observed, indicating a strong interaction between Ti 3 C 2 T x and CZS. Due to its extremely low content (Figure 4b between Ti 3 C 2 T x and CZS. The above results indicate the successful preparation of the CZS/Ti 3 C 2 T x composite photocatalyst [47]. The surface composition and chemical valence of CZS-2 were analyzed by X-ray pho toelectron spectroscopy (XPS). As shown in Figure 4a, S, Zn, Cd, C, and other elements could be detected on CZS-2. In comparison with the original CZS, slight shifts in the peaks of CZS-2 could be observed, indicating a strong interaction between Ti3C2Tx and CZS. Due to its extremely low content (Figure 4b) of Ti3C2Tx and coverage by CZS, no obvious peak could be detected on the full spectrum of the Ti element. The peaks at 454.9, 456.0, and 461.1 eV on the pristine Ti3C2Tx corresponded to Ti-C, Ti-X, and Ti-O, respectively ( Figure  4b). The C 1s of CZS-2 corresponded to the C-C, C-C-O, and C-F bonds at 284.8, 287.7, and 293.4 eV, respectively (Figure 4c). Two peaks of S 2p, at 162.0 and 168.8 eV, corresponded to S 2p3/2 and S 2p1/2 (Figure 4d), respectively. Two peaks of Cd 3d at 405.0 and 411.7 eV corresponded to Cd 3d5/2 and Cd 3d3/2 (Figure 4e), respectively. Two peaks of Zn 2p a 1044.5 and 1021.5 eV corresponded to Zn 2p3/2 and Zn 2p1/2 (Figure 4f), respectively. Com pared with the standard electron binding energy control, there was a slight shift in the peak positions of these elements, mainly due to the strong interaction between Ti3C2Tx and CZS. The above results indicate the successful preparation of the CZS/Ti3C2Tx composite photocatalyst [47]. UV-VIS diffuse reflectance spectroscopy (DRS) is widely used to investigate optical absorption of photocatalysts. In order to explore effective activity of the CZS/Ti 3 C 2 T x composite photocatalyst, DRS tests were performed in the range of 300-800 nm. As shown in Figure 5a, the CZS/Ti 3 C 2 T x photocatalyst had good light absorption ability from 300 to 500 nm, and the absorption edge appeared at about 480 nm. With increasing the amount of Ti 3 C 2 T x , the light absorption gradually increased after 480 nm. In order to further study the band structure, the Kubelka-Munk equation [48] was used to calculate the band gap: Here α, hν, and A denote the absorption coefficient, photon energy, and a constant, respectively. The calculated band gap values of CZS and CZS-2 in the original sample were 2.572 eV and 2.583 eV (Figure 5b), respectively, which also verified that CZS-2 had stronger light absorption ability. The valence band (VB) spectrum of XPS (shown in Figure 5c,d) was used to further analyze the band structure. The results show that the band gap and valence band of CZS were 2.572 eV and 1.55 eV, respectively, and the band gap and valence band of CZS-2 were 2.583 eV and 1.48 eV, respectively. The conduction band (CB) of CZS was −1.022 eV; the CB of CZS-2 was −1.103 eV according to the formula E VB = E CB + E g . Compared to CZS, CZS-2 had a more negative CB potential, which enhanced its photocatalytic reduction ability and was more conducive to the photocatalytic decomposition of the hydrogen water solution.  UV-VIS diffuse reflectance spectroscopy (DRS) is widely used to investigate optical absorption of photocatalysts. In order to explore effective activity of the CZS/Ti3C2Tx composite photocatalyst, DRS tests were performed in the range of 300-800 nm. As shown in Figure 5a, the CZS/Ti3C2Tx photocatalyst had good light absorption ability from 300 to 500 nm, and the absorption edge appeared at about 480 nm. With increasing the amount of Ti3C2Tx, the light absorption gradually increased after 480 nm. In order to further study the band structure, the Kubelka-Munk equation [48] was used to calculate the band gap: Here α, hν, and A denote the absorption coefficient, photon energy, and a constant, respectively. The calculated band gap values of CZS and CZS-2 in the original sample were 2.572 eV and 2.583 eV (Figure 5b), respectively, which also verified that CZS-2 had In order to further investigate the effect of Ti 3 C 2 T x on the charge transfer efficiency of CZS, the photocurrent response and photoluminescence maps were studied. As can be seen from Figure 6a, the photocurrent density of the CZS-2 composite photocatalyst was significantly higher than that of the original CZS sample, indicating that CZS had an improved photocurrent response and higher electron transfer efficiency after being combined with Ti 3 C 2 T x [49][50][51]. In Figure 6b, the PL emission intensity of the CZS-2 composite photocatalyst was also significantly lower than that of the original CZS. The lower emission intensity reflects the lower binding rate of the photoelectron-hole pair, which means that the combination of the photoelectron-hole pair could be inhibited after recombination, and the photogenerated carrier separation efficiency was higher [52,53]. This phenomenon is consistent with the photocurrent response diagram.
stronger light absorption ability. The valence band (VB) spectrum of XPS (shown in Figure  5c,d) was used to further analyze the band structure. The results show that the band gap and valence band of CZS were 2.572 eV and 1.55 eV, respectively, and the band gap and valence band of CZS-2 were 2.583 eV and 1.48 eV, respectively. The conduction band (CB) of CZS was −1.022 eV; the CB of CZS-2 was −1.103 eV according to the formula EVB = ECB + Eg. Compared to CZS, CZS-2 had a more negative CB potential, which enhanced its photocatalytic reduction ability and was more conducive to the photocatalytic decomposition of the hydrogen water solution. In order to further investigate the effect of Ti3C2Tx on the charge transfer efficiency of CZS, the photocurrent response and photoluminescence maps were studied. As can be seen from Figure 6a, the photocurrent density of the CZS-2 composite photocatalyst was significantly higher than that of the original CZS sample, indicating that CZS had an improved photocurrent response and higher electron transfer efficiency after being combined with Ti3C2Tx [49][50][51]. In Figure 6b, the PL emission intensity of the CZS-2 composite photocatalyst was also significantly lower than that of the original CZS. The lower emission intensity reflects the lower binding rate of the photoelectron-hole pair, which means that the combination of the photoelectron-hole pair could be inhibited after recombination, and the photogenerated carrier separation efficiency was higher [52,53]. This phenomenon is consistent with the photocurrent response diagram. Under conditions of visible illumination (UV cut-off filter, λ > 420 nm), the hydrogen evolution performance of the CZS/Ti3C2Tx composite photocatalyst was analyzed. As shown in Figure 7a, CZS with different Mxene mass ratios had good stability in the hydrogen production process for 3 h. The hydrogen production performances of CZS/Ti3C2Tx showed an obvious upward trend with increasing the amount of Ti3C2Tx in the solution using sodium sulfide and sodium sulfite as sacrificial agents. When the content of Ti3C2Tx reached 2%, the hydrogen production performance reached a maximum Under conditions of visible illumination (UV cut-off filter, λ > 420 nm), the hydrogen evolution performance of the CZS/Ti 3 C 2 T x composite photocatalyst was analyzed. As shown in Figure 7a, CZS with different Mxene mass ratios had good stability in the hydrogen production process for 3 h. The hydrogen production performances of CZS/Ti 3 C 2 T x showed an obvious upward trend with increasing the amount of Ti 3 C 2 T x in the solution using sodium sulfide and sodium sulfite as sacrificial agents. When the content of Ti 3 C 2 T x reached 2%, the hydrogen production performance reached a maximum value of 47.1 mmol g −1 h −1 , which was 1.3 times that of the original CZS sample (Figure 7b). At 420 nm, it had a higher AQY value of 27.24%. Subsequently, with increasing the amount of Ti 3 C 2 T x , the hydrogen production performance gradually decreased, indicating that excessive addition of Ti 3 C 2 T x may have a negative impact on the photocatalytic activity of CZS. An excessive amount of Ti 3 C 2 T x can result in a decrease in the concentration of the main photocatalyst CZS and also hinder the penetration of light, thereby impeding the utilization efficiency of CZS for light. Therefore, the optimum loading for Ti 3 C 2 T x is 2 wt.%. The hydrogen evolution stability of CZS/Ti 3 C 2 T x was analyzed by four repeated experiments. After four cycles of 3 h (as shown in Figure 7c), the hydrogen evolution performance of CZS/Ti 3 C 2 T x decreased slightly and the hydrogen production efficiency was 82.7%, as compared with that of the first cycle due to part of the photocatalyst covering the surface of the reaction vessel with hydrogen production, thus affecting the incident photon quantity.  Figure 8 shows the possible photocatalytic hydrogen evolution mechanism CZS/Ti3C2Tx system. For the primitive CZS, when exposed to visible light (λ > 4 after excitation, the photogenerated electrons on VB were induced to transfer to C combine with H + in water to produce hydrogen. The holes generated by the tran electrons on VB reacted with the sacrificial agents S 2− and SO3 2− on the surface of th tocatalyst. In the absence of Ti3C2Tx recombination, the photolithogenic holes an trons showed a high and rapid recombination rate. After adding the appropriate qu of Ti3C2Tx, a heterojunction could be formed. The Schottky barrier was generated CZS/Ti3C2Tx interface. Additionally, due to the higher Fermi energy level of Ti3C2T pared to CZS, photogenerated electrons tended to migrate from the conduction b CZS to Ti3C2Tx, effectively impeding the recombination of photogenerated electro pairs and thereby enhancing the separation efficiency of electron-hole pairs. In ad owing to the large dispersion area of Mxene, the agglomeration of CdZnS nanop could be effectively inhibited. As a result, with the aid of Ti3C2Tx Mxene, CZS show  Figure 8 shows the possible photocatalytic hydrogen evolution mechanism in the CZS/Ti 3 C 2 T x system. For the primitive CZS, when exposed to visible light (λ > 420 nm) after excitation, the photogenerated electrons on VB were induced to transfer to CB, then combine with H + in water to produce hydrogen. The holes generated by the transfer of electrons on VB reacted with the sacrificial agents S 2− and SO 3 2− on the surface of the photocatalyst. In the absence of Ti 3 C 2 T x recombination, the photolithogenic holes and electrons showed a high and rapid recombination rate. After adding the appropriate quantity of Ti 3 C 2 T x , a heterojunction could be formed. The Schottky barrier was generated on the CZS/Ti 3 C 2 T x interface. Additionally, due to the higher Fermi energy level of Ti 3 C 2 T x compared to CZS, photogenerated electrons tended to migrate from the conduction band of CZS to Ti 3 C 2 T x , effectively impeding the recombination of photogenerated electron-hole pairs and thereby enhancing the separation efficiency of electron-hole pairs. In addition, owing to the large dispersion area of Mxene, the agglomeration of CdZnS nanoparticles could be effectively inhibited. As a result, with the aid of Ti 3 C 2 T x Mxene, CZS showed an improved visible-light-driven photocatalytic hydrogen evolution.

Conclusions
In summary, we have constructed and characterized an efficient photocatalyst of CZS nanoparticles overlaid on cocatalyst Ti3C2Tx nanosheets using an in situ growth method. The photocatalytic hydrogen production performance of CZS can be effectively improved by adding an appropriate amount of Ti3C2Tx (2% mass ratio), which can construct heterojunctions to block the recombination of electrons and holes induced by photogeneration. Under visible light irradiation, it reached a high photocatalytic hydrogen production value of 47.1 mmol g −1 h −1 . It could also maintain a relatively stable hydrogen production over a long period of catalysis. The main reason for the success of the modification is that the Schottky barrier is conducive to the diffusion of electrons and inhibits the recombination of electron-hole pairs induced by photogeneration. At the same time, the zinc cadmium sulfide nanoparticles uniformly covered the Ti3C2Tx nanosheets, also improving the utilization rate of light and relieving the agglomeration effect of the particles, thereby achieving a high hydrogen production performance. This work can provide some valuable guidance and ideas for selecting cocatalyst modification of CZS to improve the separation of electron-hole pairs and inhibit the recombination of electron-hole pairs.

Conclusions
In summary, we have constructed and characterized an efficient photocatalyst of CZS nanoparticles overlaid on cocatalyst Ti 3 C 2 T x nanosheets using an in situ growth method. The photocatalytic hydrogen production performance of CZS can be effectively improved by adding an appropriate amount of Ti 3 C 2 T x (2% mass ratio), which can construct heterojunctions to block the recombination of electrons and holes induced by photogeneration. Under visible light irradiation, it reached a high photocatalytic hydrogen production value of 47.1 mmol g −1 h −1 . It could also maintain a relatively stable hydrogen production over a long period of catalysis. The main reason for the success of the modification is that the Schottky barrier is conducive to the diffusion of electrons and inhibits the recombination of electron-hole pairs induced by photogeneration. At the same time, the zinc cadmium sulfide nanoparticles uniformly covered the Ti 3 C 2 T x nanosheets, also improving the utilization rate of light and relieving the agglomeration effect of the particles, thereby achieving a high hydrogen production performance. This work can provide some valuable guidance and ideas for selecting cocatalyst modification of CZS to improve the separation of electron-hole pairs and inhibit the recombination of electron-hole pairs.